This invention relates generally to safety interlock devices utilized to prevent human access to direct or collateral radiation upon removal or displacement of the interlocked portion of a protective housing enclosing such radiation. More particularly, the invention provides a mechanical safety interlock device including a shutter, for laser products, which device is defeatable but precludes removal or displacement of the interlocked portion of the protective housing upon failure or defeat of the interlock to prevent access to the laser radiation.
Products utilizing hazardous radiation must be provided with protective housings to prevent access to direct or collateral exposure to such radiation. For safety considerations such products further must be provided with safety interlock devices to prevent human access to the hazardous radiation upon removal or displacement of a portion of the protective housing as would occur for the purpose of gaining access to the interior of such housing. In addition, the interlocked portion of the protective housing must be provided with means to preclude removal or displacement thereof upon failure of the interlock to prevent human access to the direct or collateral radiation in excess of tolerated amount. Ordinarily, the safety interlock devices are permitted to be defeatable but if defeated, there must be provided a visible indication of defeat as well as means to preclude replacement of the removed or displaced portion of the protective housing so long as the safety interlock device remains defeated.
For a given product, electrical interlock switch devices are well known and widey used. These may be located in proximity to access doors and panels for actuation by the door, the panel or a latch which may actuate a leaf spring, plunger or push-button. The switch may be located in a power or control circuit or may operate a solenoid shutter or baffle in order to prevent human access to the radiation upon movement of the protective portion of the housing or latch.
Electrical interlocks are subject to failure modes involving the failure of switch contacts to make-or-break upon actuation or the failure of another component although the switch itself performed properly.
Plug type interlock devices such as used on television receivers are familiar. In the case of television receivers, the primary electrical power is delivered to the product by a plug which is integrally mounted to the portion of the housing that is to be removed for access to the interior of the product. Removal of the back cover of the receiver, removes all electrical power. Other types of plug type interlock devices may mount only a portion of an electrical circuit on the removable portion of the housing while other types may employ a single prong plug to complete a circuit. Location must be suitable and, of course varies with product. Short circuits and/or other component failures are failure mode considerations which must be taken into account.
For many products, especially those which utilize hazardous radiation such as lasers for example, they may be more suitably protected with interlock devices that are mechanical in operation using baffles or shutters which block the radiation from entering the area made accessible by removal of the housing, which frustrate a laser cavity, or remove an integrally mounted optical component, insert, attenuator, etc. Sturdiness of design and component failure modes must be considered.
In providing for an effective interlock device meeting safety standards and requirements, including government regulations, consideration must be given to all modes of possible failure which might fail to prevent access. Sticking of a push button, or leaf actuator, shorting or welding of switch contacts, failure of another component in an interlock circuit, or mechanical jamming of a shutter, for example may result in failure to preclude human access to the radiation upon removal or displacement of the protective housing or part thereof.
By their very nature, electrical interlock devices must be supplemented by provision of mechanical means such as electrically operated latches, which keep the housing secured until the electrical circuit is interrupted by opening of the contacts or of the interlock switch or the cessation of the radiation. Plug type interlocks are more effective but must depend upon proper design to preclude electrical or mechanical failure.
Another consideration in addition to possible operational failure, is the fact that any interlock system regardless of its complexity can be defeated by persons with sufficient motivation and cleverness. In fact, the system often is intended to permit defeat. Defeat may be occasioned by pushing a button or leaf spring, installing a clip lead across obvious switch terminals, pulling out of a "pull to defeat" plunger, placing tape over a push-button to hold the same in defeated condition, and the like. However, it is essential that if defeated, the safety interlock must provide a display of such defeat. The indication must be clear in its meaning and appropriate for the product and its operating environment. Visibility of a piece of tape say, holding a leaf spring depressed or a push-button recessed, may not be clear enough to satisfy safety regulations. Plug type interlock devices require the use of a by-pass for defeat. In the case of television receivers, an external power service cable can be utilized to permit operation for testing with the cover removed. Visibility is evident.
Mechanical interlocks may incorporate integral flags which indicate defeat or defeat may be accomplished by the installation of a tool which itself gives indication of defeat.
It is essential that the interlock device must be provided with means whereby replacement of the removed or displaced portion of the protective housing is precluded so long as the interlock device is defeated and remains in defeated condition. Reset means for electrical interlock devices may be provided but then cannot operate until the displaced or removed portion of the protective housing is replaced. So long as by-pass means is utilized to overcome plug type interlock devices, re-installation of the back cover, for example, is not possible. Mechanical interlock devices must be provided with means whereby replacement of the removed portion of the housing which triggered the interlock cannot be effected so long as the interlock device is defeated.
One type of product for which a safety interlock device is required involves a cabinet within which a laser beam is housed for operation through a passageway or port. Shutter means is provided to prevent passage of the laser beam through said port. Access to the laser radiation is gained by opening a door to the cabinet.
The safety interlock device desirable for such product must block the laser beam when a portion of the protective housing of the product is removed or displaced, as when the access door to the cabinet is opened. In addition, replacement of the door must be rendered impossible while the safety interlock is defeated and as well, means must be provided to preclude removal or displacement of the door, for example, upon failure of the safety interlock to prevent access to the laser radiation.
The interlock device thus provided must meet all regulatory standards concerning safety interlock systems for laser products and those other products utilizing dangerous or hazardous radiation, particularly those standards that entitle the producer to avoid required placement of supplementary safety devices and/or warning devices such as key switches, special alarms, fail-safes, etc. which would increase the cost of the product or deter users with notices and other alarm raising means.